Myeloid lineage cells (macrophage/microglia) are central to the ability of primate lentiviruses (HIV/SIV) to establish a viral reservoir in the central nervous system (CNS) and the presence of such a reservoir underscores the neuropathogenic manifestations of virus infection. However, beyond their role in CNS infection, it is not known whether monocyte/macrophage are even necessary for infection of the host by HIV/SIV and definitive experiments demonstrating such an essential role are lacking. Our studies on HIV- myeloid cell interaction have revealed the existence of viral determinants that specifically operate in monocyte/macrophage. These determinants are essential for the ability of the virus to invade the cell and for the ability of the virus to maintain a persistent infection. Based on this, we hypothesize that myeloid cells are essential for infection of the host and propose that strategies that truncate myeloid cell infection will fundamentally impact the course of infection by curtailing viral persistence, neuroinvasion and pathogenicity. To pursue this hypothesis, we propose the following specific aims: Aim 1: Examine the in vivo phenotype (replication, persistence, neuroinvasion) of SIV mutants incapable of infecting myeloid lineage cells. Our research has defined a viral determinant (Vpr/Vpx) that is essential for the infection of myeloid cells by HIV/SIV yet that is dispensable for infection of CD4+ T lymphocytes. Infection of macaques with these mutants will allow us to define how the myeloid cell reservoir shapes the course of viral replication, persistence and pathogenicity in vivo. Aim 2: Evaluate the contribution of myeloid reservoirs to chronic virus infection through the use of pharmacologic agents that truncate infected macrophage survival. Our research has established a mechanism by which monocyte/macrophage resist viral cytopathic effects that relies on the production and signaling of the survival cytokine monocyte colony stimulating factor (MCSF) by infected macrophage. We have determined that agents that interfere with the function of the MCSF receptor restore the sensitivity of macrophage to virus-mediated cytopathicity. We will examine whether pharmacologic inhibition of survival cytokine signaling curtails viral persistence in the macrophage phase of SHIVDH12 infection in macaques. Aim 3: Define the cellular factors that regulate myeloid cell infection by primate lentiviruses. Antiviral restrictions such as Apobec 3G and Tetherin have generated intense interest in the field. Our research reveals the existence of a novel antiviral factor that is specifically expressed by myeloid cells. This restriction potently inhibits myeloid cell infection by HIV/SIV and the viral Vpr/Vpx proteins counteract the antiviral activity of this restriction. We will use biochemical and genetic approaches to characterize the cellular activity that regulates myeloid cell infection. Studies outlined in this proposal will define the role of the myeloid cell reservoir in the biology of primate lentiviruses and in the interaction of primate lentiviruses with their hosts. This information will be used to devise strategies with which to prevent the establishment of HIV-1 infection in the host as well as strategies to curtail the chronic nature of virus infection.